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NAT'L INST. OF STAND & TECH

NBS SPECIAL PUBLICATION 260-44

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lOOMMERCE

Standard Reference Materials:

PREPARATION AND USE OF

SUPERCONDUCTIVE

FIXED POINT DEVICES,

SRM 767

NATIONAL BUREAU OF STANDARDS

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APR 9 %

U6^ Standard Reference Materials:

Preparation and Use of

Superconductive Fixed Point Devices, SRM 767

J. F. Schooley, R. J, Soulen, Jr., and

G. A. Evans, Jr.

Heat Division

Institute for Basic Standards

^J' National Bureau of Standards

Washington, D.C. 20234

U.S. DEPARTMENT OF COMMERCE, Peter G. Peterson, Secreiary

NATIONAL BUREAU OF STANDARDS, Lawrence M. Kushner, kcWng Director,

Issued December 1972

Library of Congress Catalog Card Number: 72-600339

National Bureau of Standards Special Publication 260-44

Nat. Bur. Stand. (U.S.), Spec. Publ. 260-44, 35 pages (Dec. 1972)

CODEN: XNBSAV

For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402

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PREFACE

Standard Reference Materials (SRM's) as defined by theNational Bureau of Standards are "well-characterized mate-rials, produced in quantity, that calibrate a measurementsystem to assure compatibility of measurement in the nation.

"

SRM's are widely used as primary standards in many diversefields in science, industry, and technology, botlj within theUnited States and throughout the world. In many industriestraceability of their quality control process to the nationalmeasurement system is carried out through the mechanism anduse of SRM's. For many of the nation's scientists and tech-nologists it is therefore of more than passing interest toknow the details of the measurements made at NBS in arrivingat the certified values of the SRM's produced. An NBS seriesof papers, of which this publication is a member, called theNBS Special Publication - 260 Series is reserved for thispurpose

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Inquiries concerning the technical content of thispaper should be directed to the author (s). Other questionsconcerned with the availability, delivery, price, and soforth will receive prompt attention from:

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V

CONTENTS

PAGE

I. INTRODUCTION 1

II. SAMPLES 3

III. EXPERIMENTAL APPARATUS AND METHOD 5

A. Apparatus 5B, Experimental Procedure 7

IV. ANALYSIS OF DATA AND TEMPERATURE MEASUREMENT 8

A. Data Analysis 8

B. Temperature Measurement 13

V. USE OF SRM 767 15

VI. APPENDIX A 18

VII. REFERENCES 20

LIST OF TABLES

PAGETable No .

I. Assays of Bulk Starting Materials for Super-conductive Thermometric Fixed Point Devices(ppm by weight) 3

II. Temperature Dependences of Germanium Thermo-meters (values in parentheses have changedduring the experiments described in thisreport) 9

III. Summaries of AT Values, by Element, in mK,Before and Afte? Adopting a Grease-Plus-Conductive-Varnish Mounting Technique 12

IV. Assigned Transition Temperatures of Super-conductive Samples, Derivations of the AssignedValues, Uncertainties in the Assignments, andExperimental Reproducibilities as Fixed Points 14

vi

LIST OF FIGURES

PAGE

Figure No .

1. The experimental cryostat 21

2. Schematic of the copper disk and samplemounting assembly 22

3. Block diagram of the transition measurementscheme, illustrating the definitions of thetransition width, W. and of the transitiontemperature, T 23

c

4 . Schematic drawing of the mutual inductancecircuit 24

5. Schematic drawing of the resistance measure-ment circuit 25

vii

PREPARATION AND USE OF

SUPERCONDUCTIVE FIXED POINT DEVICES

SRM 767

by

J. F. Schooley, R. J. Soulen, Jr., and G. A. Evans, Jr.National Bureau of Standards

Department of CommerceWashington, D. C. 20234

The preparation, testing, and use of SRM76 7 devices are described. These devicesincorporate samples of lead, indium, alumi-num, zinc, and cadmium within a mutualinductance coil pair. These elementsbecome superconductive at temperatures near7.2 K, 3.4 K, 1.2 K, 0.85 K and 0.5 K,respectively, and the transition midpoints,when attained by observing the samplemagnetic susceptibilities in negligiblysmall magnetic fields, provide thermometricreference points which are reproducible to+ 1 mK.

Key words: Aluminum; cadmium; cryogenics;indium; lead; magnetic susceptibility;superconductive transition temperature;superconductivity; thermometric fixedpoints; zinc.

I . INTRODUCTION

In scientific and engineering work, temperature measure-

ments below 20 K can be referred to a variety of thermometric

fixed points and temperature scales. The International

Practical Temperature Scale of 1968 [1] recognizes the triple

point and two boiling points of equilibrium hydrogen as

fixed points, and it defines temperatures from 13.81 K

upwards in terms of the resistance of standard platinum

1

resistance thermometers. In addition, it recommends the

1958 '^He and the 1962 ^He vapor pressure scales for use

between 0.2 K and 5.2 K. Furthermore, it appears likely

that a temperature scale based on the velocity of sound in

pure gases [2] eventually will be recognized as a standard

from about 2 K to 30 K. Still other temperature scales,

such as one based on the thermal noise in a resistance [3]

and another involving the angular distribution of radiation

from oriented nuclei [4] , appear to be quite promising at

lower temperatures

.

However, the realization of each of these fixed points

and temperature scales is a challenging problem in the

laboratory. With the exception of the ^He and the '*He

vapor pressure scales, none has been used with any regular-

ity in actual experimental cryostats . With the advent of

the ^He dilution refrigerator [5] , moreover, the cooling bath

which determines the temperature of the experimental chamber

is no longer a pure liquid, but rather is a variable mixture

of the two isotopes of helium, for which no ready vapor

pressure — temperature relation is at hand.

In order to provide reliable temperature information,

various experimenters use semiconductor resistance, paramag-

netic susceptibility, thermocouple, nuclear resonance and

other theinnometers . These have the common feature that

they must be calibrated so that the temperature-dependent

observable can be related to an accepted temperature scale;

in many cases, this must be done for each experiment be-

cause the device is not reproducible after warming to room

temperature

.

One means of providing a convenient in situ temperature

calibration involves the observation of the superconductive

transitions in various metals. Since T values are oftenc

quite sensitive to impurities, it appeared resonable to

examine the purest stocks available. Preliminary studies [6]

2

indicated that superconductive transitions reproducible

to + 1 mK might be obtained for pure samples of lead

(T^ 'V 7.2 K) , indium (T^ ^ 3.4 K) , aluminum (T^ ^ 1.2 K) ,

zinc (T 0.8 K), cadmiiim (T 0.5 K), and iridiumc c

(T 'v. 0.1 K). The superconductive transitions in higher -oalloys were quite broad, so that no further attempt

has been made to use them in thermometric fixed point devices.

A search for sources of pure, yet inexpensive supplies of

the elements listed above indicated that only the first five

were readily available. This report contains a discussion

of the preparation and use of devices containing these

elements

.

II . SAMPLES

Bulk quantities of high-purity aluminum, zinc, and

cadmium were available from the Office of Standard Reference

Materials. Ingots of lead in indium were obtained commer-

cially. The following table lists the assays available with

these materials

:

Table I. Assays of Bulk Starting Materialsfor Superconductive ThermometricFixed Point Devices (ppm by weight)

ZINC LEAD CADMIUM ALUMINUM INDIUM

;rm 6 8 2 HPM 9284 SRM 746 HPM 5831 JK 762

CI < .5 Cu .2 K < 4 Si 1. Pb 3

0 < .5 Cd .1 Na < 3 Cu .5 Tl 3

Si < .5 Fe .1 0 < 2 Mg .5 Sn 1

Ca < .2 Si .1 Pb .8 Ca .2 Cd < 1

Na < .2 Tl .1 Ca < .6 Cr .2 Fe < 1

Ti < .2 Bi < .1 Cr .4 Ag .1 Cu nf

Cd .1 Ca < .1 C < .1 Ga nf

3

Table I (continued)

ZINC LEAD CADMIUM INDIUMSRM 682 HPM 9284 SRM 746 JK 762

Fe .1 Mg < .1 Mg .1 Ni nf

K < .1 Ag < .1 CI .2 Ag nf

Mg < . 1 Zn .

1

Ni < .1 As < .1

Rb < .1

(Fe, Mn-not determined)

The preparation of fixed-point samples of Pb, In, Zn,

and Cd was begun by casting the high-purity ingots, under

high vacuum, into rods 1 cm in diameter and 25 cm long.

These rods were cut into slugs 1.5 cm long and then etched

with nitric acid to remove possible contamination. Sub-

sequently, the slugs were handled with clean tweezers.

Before the slugs were placed in pyrex molds for specimen

casting, they were rinsed three times in distilled water

and dried.

The casting of the slugs into rods 1.5 mm in diameter

and 15 cm long was done under high vacuum while heating

the pyrex molds to the melting point of the particular

material being cast. In order to force the molten slug

into the capillary, it was often necessary to isolate the

vacuum pump from the mold and to introduce an atmosphere

of helium. The helium was purified by passing it through

a zeolite trap at 77 K.

With the exception of Zn, the glass molds were etched

away from the rod with hydrofluoric acid. The Zn samples

were not prepared this way because hydrofluoric acid

attacks the metal; instead, they were extracted by breaking

the glass mold with a wooden mallet. The specimens cut

from the 1,5 mm rod were then filed, rounding the ends to

reduce any effects on the measurement of the superconductive

4

transitions. The samples were etched in a solution of nitric

acid and rinsed three times in distilled water. The Pb

samples received an additional alcohol rinse, as this seemed

to retard oxidation.

To avoid excessively broad (20-40 mK) transitions,

it was necessary to anneal the Zn and Cd samples; however,

it was not necessary for the Al, Pb, and In specimens.

The Zn specimens were annealed for 48 hours at 395-398 °C

and the Cd samples were annealed for 48 hours at 30 8-311 °C.

Both Zn and Cd samples were sealed in pyrex tubes with two-

thirds of an atmosphere of purified helium to prevent subli-

mation.

The Al samples were made by tying six 3 cm long, 0.5 mm

diameter wires of OSRM HPM 5831 Al into a bundle with nylon

thread

.

III. EXPERIMENTAL APPARATUS AND METHOD

A . Apparatus

The apparatus used in this work was a ^He-'*He dilution

refrigerator with a countercurrent capillary heat exchanger,

in which a copper platform was provided for the simulta-

neous measurement of up to five germanium resistance thermo-

meters and of as many as five sets of superconductive samples.

The single-heat-exchanger dilution refrigerator has

been discussed in the literature, and its operating charact-

eristics are reasonably reliable [5] . Design features of this

cryostat which are particularly relevant for these experi-

ments are shown in schematic form in Fig. 1.

In order to provide an environment with thermometric

precision and reproducibility of one millikelvin at temper-

atures ranging from 0.5 to 20 K, it was necessary to employ

a set of five germanium resistance thermometers monitored

5

by a set of the superconductive fixed point samples. The

five resistance therrnometers , five superconductive devices

and a resistive heater were mounted on the copper platform

shown in Fig. 1.

Individual samples were inserted into a copper stud,

as shown in Fig. 2. The following procedure was developed

for mounting to ensure thermal equilibrium and mechanical

rigidity between the samples and the stud; stopcock grease

was inserted into the mounting hole before the sample was

placed therein, and an electrically conductive varnish-silver

powder mixture was brushed on afteirward. It was found in

early experiments that several samples could be included in

a single stud without noticeable interaction; in fact, holes

for extra samples were drilled in the studs in the event

that other materials might eventually prove useful for

thermometric fixed points.

As indicated in Fig. 2, the transitions were observed

by means of a mutual inductance coil pair. In these measure-

ments, the primary coils on each of the five devices were

connected in series and the five secondary coils had one

common lead; thus the measurement of twenty-five individual

samples required but eight electrical leads.

The germanium thermometer resistances were obtained by

four-lead dc potentiometry , in which the voltage across a

standard 1000 9, resistor and the voltage across the one

resistance thermometer electrically in series with it were

measured alternately. The voltage could be resolved to 0.1

yV and, for the measuring currents used, the resultant

temperature resolution was <_ 0.1 mK.

Superconductive fixed point devices were made part

of a mutual inductance bridge circuit, which was monitored

by a phase-sensitiVe detector. The circuit is shown in

block form in Fig. 3, and is discussed in Appendix A. The

device coils were operated at 400 Hz with a magnetic field

6

amplitude of a few hundred nT. The earth's magnetic field

was reduced by external Helmholtz coils to less than 1 yT.

This method of observing the superconductive transitions

was selected primarily because of its convenience. It has

the added advantage, however, of avoiding the introduction

of strains or impurities which might accompany the placing

of electrical contacts on the sample for resistance measure-

ments. Finally, the magnetic inductance method has been

shown to produce values of T^ within one millikelvin of

those obtained from resistivity and heat capacity measure-

ments [7] .

B. Experimental Procedure

Before attempting to stabilize the platform at a

temperature point, the dilution chamber was first stabilized

a few tens of millikelvins below that point. For tempera-

tures below about 1 K, this procedure required operation

of the dilution refrigerator at a reduced ^He flow rate

and the application of up to one milliwatt of heat to the

dilution chamber. The thermal link to the dilution chamber

then brought the platform to a temperature slightly below

that desired with in a few minutes.

To reach the temperature of the appropriate supercon-

ductive fixed point, a manually variable direct current

was then applied to a resistor mounted on the center of the

platform while the progress of the superconductive transi-

tion was followed on the phase-sensitive mutual inductance

bridge detector meter or on an X-Y recorder connected to it.

Varying the heater current stabilized the platform at the

temperature characterized by the midpoint of the supercon-

ductor's inductive transition. To avoid errors due to

possible super-cooling effects, the transition midpoint was

always approached from the low-temperature side, and the

7

earth's field compensation was often checked before pro-

ceeding with the resistance measurements [8].

One result of using the dilution chamber for rough

temperature control was that less than 10 microwatts of

power was generated in the platform resistor during most

measurements; thus the possibility of thermal gradients due

to large heat flow in the platform was reduced. A second

advantage in using this method is that the electrical leads

to the platfoirm components pass through two nearly isothermal

"anchors", considerably reducing heat flow to or from the

components through their leads. For similar reasons, the

temperatures of the pumped "^He bath and of the ^He evapor-

ator were adjusted to consistent values each time measure-

ments were made at a given platform temperature.

Once stability was obtained at a given platform temp-

erature, the appropriate germanium resistors were measured

with one or more current settings.

IV. ANALYSIS OF DATA AND TEMPERATURE MEASUREMENT

A, Data Analysis

Analysis of the individual superconductive transitions

involved an evaluation of the width of the transition and

of the temperature of its transition midpoint in relation

to other samples of that kind. For these purposes, it was

necessary to know dR/dT, the temperature dependence of the

resistance of the appropriate germanium thermometers , at

each of the superconductive transition temperatures

.

In an early set of measurements , five devices were

measured many times, while the effects of thermal cycling,

resistor measuring current variations and mutual inductance

bridge parameter variation were examined. The resistance

values of the appropriate resistors were measured at the

8

midpoints of the superconductive transitions of each sample.

From these measurements, an average resistance, R^^^„, of

each resistor was obtained for each fixed point for which

the resistor was a useful thermometer. The resulting values

of Rj^yg are listed in the last column in Table II. Resistors

Table II. Temperature Dependences of GermaniumThermometers (values in parentheseshave changed during the experimentsdescribed in this report)

Thermometer dR Q R nNumber Temperature , K dT ' mK AVE '

1394 7.2 (lead) 0.08 (314.45)

1395 7.2 0.08 346.31

2412 7.2 0.08 (339.65)

1394 3.4 (indium) 0.8 1347.0

1395 3.4 0.8 1470.4

2412 3.4 0.8 (1580.3)

1394 1.2 (aluminum) 60. 20,015.

1395 1.2 70. 21,490.

2412 1.2 70. (22,193.)

452 1.2 1.7 (1,104.0)

452 0.84 (zinc) 5.7 (2,129.3)

1403 0.84 0.25 123.21

452 0.5 (cadmium) 40. (7,321.)

1403 0.5 0.4 206.65

9

139 4 and 1395 were calibrated against the NBS 2-20 K acous-

tic temperature scale [2] , readily yielding values of dR/dT

at the lead and indium points. Resistor 452 was calibrated

against T^^ [9] from 0.95-2 K in a separate experiment, and

the calibration was extended to 0.4 K by means of cerous

magnesium nitrate paramagnetic salt thermometry [10] ; these

measurements yielded values of dR/dT for resistor 452 at

the aluminum, zinc, and cadmium points. In order to obtain

the remaining values of dR/dT, it was sufficient to apply

small magnetic fields to the fixed-point samples, reducing

the temperature of the superconductive transitions slightly,

and to measure the resistances of the appropriate resistors

once again, obtaining the temperature shifts from values

of (dH /dT) in the literature,c i c

Once the resistor temperature dependences shown in TableII were determined, it was possible to obtain values for

deviations of individual sample T^'s from the average T^

for that element, and to evaluate the width of each transi-tion as well. The former quantity, which was designatedAT^^ was obtained from the relation

AT^ = AR [dR/dT]

where AR was simply the difference between the resistance

at the sample T and Rt.^^. The width, W, was obtained from^ C AVEthe X-Y recordings of the transitions by evaluating the

resistances at which the superconductive-to-normal and the

normal-to-superconductive transitions were 80-9 0% complete,

as shown in Fig. 3. Once again, if AR refers to the

resistance difference thus obtained, a simple ratio

W = ARldR/dT]"-*-

yields the desired quantity.

It should be clear from the method used to evaluate

W that any hysteresis exhibited by the sample would be

included in its measured width. However, such effects.

10

when present, rarely exceeded one millikelvin

.

The analysis of the fixed-point devices has been

complicated by changes occurring in the resistance thermo-

meters; in fact, the existence of these changes has shown

the usefulness of thermometric fixed points. In a typical

case of this kind, the AT^ values of all the samples of

one element would be relatively large and similar to each

other, according to measurements made with one resistor.

Because at least two resistors were in use at each fixed-

point temperature, this problem was easily interpreted.

Usually, the five sample AT^ values measured with all other

resistors were less than about one-half millikelvin and the

five values measured with the deviant resistor were clustered

about a relatively large value; even though only two resistors

might be in use at the fixed point, the fact that all five

samples gave similar indications was convincing evidence

that the anomalous results were due to a shift in the

resistance-temperature relation of the corresponding resistor.

For example, we have shown that resistor 1394 has shifted

from time to time by as much as 2 mK at the lead point; the

shift followed the warming of the apparatus to room temper-

ature. (It may be worth noting, however, that the resistor

itself had not been remounted, nor had its leads been

knowingly disturbed, during these times). A larger shift

has occurred once in resistor 452, referring all of the fixed-

point data to new, evidently reproducible resistance values

111] . Resistor 2412 has suffered a drastic and apparently

fatal change in its behavior, as its fixed-point values are

no longer reproducible.

Recurring anomalies in the lead and aluminum AT^ values

led to a modification in the procedure used to fasten the

samples into the Cu stud. We found that AT^ values of

perhaps one-fourth of over three dozen of these samples were

greater than 1 mK, and poor thermal response often appeared

11

during the transition recording. Upon microscopic exami-

nation of the device, it was found that the solvent used in

the anchoring varnish apparently evaporated in such a way

as to force most of the varnish out of the annular space

between the sample and its mounting hole. When stopcock

grease was substituted' for the varnish, the problem

disappeared, and no anomalous lead or aluminum AT^

has since been observed in over three dozen measurements

on these samples. Inasmuch as the AT^ values observed in

these experiments so far give a reasonable picture of the

overall experimental uncertainty of the fixed-point measure-

ments, summaries of these values are shown in Table III.

The reduction in average AT^ values accompanying the change

in mounting procedure can be seen readily. In fact, only

two of eight dozen measurements made after the mounting

procedure change have yielded AT^ values as high as 1 mK.

Table III. Summaries of AT Values, by Element,in mK, Before and After Adopting aGrease -Plus-Conductive-Varnish Mount-ing Technique

A, Varnish Only

Average Pb (22) * In (17) Al(18) Zn(17) Cd(18)

AT .63 .28 .77 .27 .42c

AT +.56 -.14 -.28 0 +.35c

B. Grease and Conductive Varnish

AT .32 (24) .15 (17) .28 (15) .22 (20) .30 (20)c

AT +.24 -,06 -.09 +.21 +.27c

*Numbers in parentheses are the numbers of specimens meas-ured .

12

B. Temperature Measurement

Values of the various transition temperatures were

derived from paramagnetic salt thermometry in conjunction

with the ^He vapor pressure-temperature scale [9] or with

the NBS 2-20 K acoustical temperature scale [2]. In the for-

mer case, the set o-f germanium resistors was calibrated at

several temperatures against readings taken on a ^He vapor

pressure bulb, and the paramagnetic susceptibility of a cerous

magnesium nitrate single crystal sphere was used to evaluate

the Zn and Cd transition temperatures. The germanium resis-

tances corresponding to the Al transition were evaluated

directly against the ^He bulb temperature. The Pb and In

transition temperatures were obtained by paramagnetic salt

thermometry interpolation between points of germanium resis-

tors which had been calibrated on the NBS 2-20 K scale.

These derivations are indicated in Table IV.

A summary of the transition temperatures assigned to

the superconductive fixed-point device samples by this

procedure is given in the second column of Table IV. The

uncertainties in assigning the appropriate temperature scale

values to the respective fixed-point resistances are shown

in the fourth column, and the experimental average magnitude

of AT is listed in the last column,c

13

Table IV. Assigned Transition Temperatures ofSuperconductive Samples, Derivationsof the Assigned Values, Uncertaintiesin the Assignments, and ExperimentalReproducibilities as Fixed Points.

Assignment ExperimentalAssigned Uncertainty Reproducibility

Element T (K) Derivation (mK) (mK)

He v.p. Tg2

Lead 7.201 (NBS 2-20 K) + 2.5 0.32salt thermometry

Indium 3-. 416^ (NBS 2-20 K) + 1.5 0.15salt thermometry

Aluminum 1.174^ Direct Calib. - 2. 0.286 3-

Zinc 0.844 T^^ + salt thermo- 1.5 0.2 252 .metry

Cadmium 0.515 T^_ + salt thermo- 2.5 0.306 2,metry

It should be emphasized that the limits of error shown in

Column 4 of Table IV indicate primarily the difficulty of

assigning temperature scale values to the appropriate

germanium resistances, while Column 5 reflects the experiment-

al reproducibilities of the various samples as thermometric

fixed points. No attempt has been made to estimate the

deviation of the two temperature scales from the thermo-

dynamic scale; there is evidence that both scales deviate

to some extent from thermodynamic temperatures , but of

course, these experiments cannot disclose such deviations.

The transition temperature assignment uncertainties shown

in Table IV can be expected to decrease as the corresponding

T values continue to be measured on the defining scales;c

the uncertainties not shown in Table IV but present none-

theless due to uncertainties in the relevant temperature

scales can be reduced by inclusion of the devices in experi-

14

ments of the kind mentioned in the introduction to this

report. It is the fond hope of the present authors that

such experiments will soon take place.

V. USE OF SRM 767

SRM 76 7 as released by NBS Office of Standard Reference

Materials consists of a set of five samples (Pb, In, Al, Zn,

and Cd) varnished into a copper stud, as shown in Fig. 2,

and enclosed by a bakelite cover and a mutual inductance

coil set. Overall, the device is about 1.5 cm diameter

by 4 cm long. An identifying serial number is located

at the top of the protective cover.

The user should be particularly aware of three con-

siderations in placing SRM 767 in service. First, the

copper stud should be brought to thermal equilibrium with

the experiment or with the thermometer which is to be

calibrated. This can be accomplished by connecting both

to a solid copper block. The SRM 76 7 copper mounting stud

terminates in a 6-32 thread about 5 mm long, and a matching

tapped hole should be provided in the sample block. A light

coating of stopcock grease placed on the threads of the

mounting stud and a light tightening pressure (simply with

the fingers) will form an adequate thermal connection between

the mounting stud and the sample block. In addition, the

four electrical leads to the measuring coils should be

varnished or greased to the sample block over a length of

perhaps 5 cm to avoid heat influx to the device via that

avenue

.

A second consideration in using the device is control

of the ambient magnetic field. In order to avoid depressing

the transition temperatures of the superconductive samples

and probably introducing hysteresis due to supercooling, the

user should reduce the ambient magnetic field to 1 yT or

15

less. This can be accomplished by using three mutually

orthogonal Helmholtz pairs outside of the apparatus , provid-

ing that no extensive ferromagnetic or superconductive

layer intervenes. (An example of an interfering super-

conductive layer would be a vacuum jacket tinned with 50/50

lead-tin solder and enclosing the sample area. Such a

cylinder will become superconductive slightly above 7 K,

and will trap the magnetic flux inside it at the time of

its transition. Subsequent adjustment of external earth's

field compensation coils would not alter the trapped inter-

nal magnetic field.) Another method of removing the earth's

field involves the use of ferromagnetic shielding using mu-

metal or its equivalent. Carefully made and carefully used,

such shielding can reduce the ambient magnetic field to

values well below 1 yT, but it is well to use it only in a

situation where the remanent field can be measured. If the

ambient field must be tolerated in the experimental chamber,

its value must be measured, and the various fixed-point

values must be reduced by the appropriate (dH^/dT)^^ values.

In addition to correcting the T^, the user should note that

supercooling of the fixed-point samples is likely to occur

in the earth's magnetic field. If it does, then only the

superconductive-to-normal transition midpoint will correspond

to T , and therefore (as will be noted below) T must bec c

reached in warming

.

The third consideration relevant to the use of SRM 76 7

is that an adequate mutual inductance measuring circuit

should be used. The circuit used in the present measure-

ments is shown in block form in Fig. 3 and in more detail

in Appendix A. It is not necessary to use a very sophis-

ticated measuring circuit, but care should be taken to

restrict the measuring field to 1 yT, (which implies a

primary current of less than 20 yA) and to obtain a stable

circuit output. In most applications, a given T^ can be

16

reached by warming the sample block while observing the

mutual inductance bridge imbalance on a meter. Having

ascertained the meter readings corresponding to the limits

of the transition, the operator can maintain by warming

the sample block a second time until the meter shows that

the transition midpoint has been reached. The heating rate

should be sufficiently slow that the superconductive-to-

normal transition is not completed. By maintaining the

heating rate at a suitable value, the operator can maintain

for an indeterminate length of time. It should be noted,

however, that drift in the measuring circuit output can

mislead the operator; indeed, a drift of one-half the tran-

sition width in the monitoring circuit can result in failure

to maintain T at all.c

In siimmary, SRM 76 7 is a four-lead mutual inductance

device which can provide millikelvin reproducibility at

each of five cryogenic temperatures, if proper care is taken

to ensure good thermal contact between the device and an

experiment or a thermometer, to cancel or to measure the

earth's magnetic field, and to provide a suitable current

amplitude and output stability in the measuring circuit.

17

VI . APPENDIX A

THE TRANSITION MEASURING CIRCUITS

The mutual inductance and thermometer resistance

circuits are shown in Figs. 4 and 5, respectively, in more

detail than in Fig. 3.

The mutual inductance measurement was based on the

well-known Hartshorn's bridge circuit. The phase-sensitive

detector supplied a 400 Hz signal to the bridge circuit

through an isolation transformer, shown as A in Fig. 4. The

standard inductor B was chosen so that the sample coil mutual

inductance (about two millihenries) was within the useful

range of the ratio arm transformer inductance, C. Switches

D and E provided for reversal of the inductive and resistive

components of the sample secondary coil voltage. Transfor-

mer F isolated the secondary circuit form the detector ground.

Typically, the sample coil primary current was a few micro-

amperes . The mutual inductance change which occurred when

a particular sample became superconductive was about ten

microhenries; in this circuit, the resulting detector signal

was perhaps five microvolts.

It is quite feasible to observe the superconductive

transitions with simpler mutual inductance equipment, and

work is in progress to develop an economical but stable

circuit.

The resistance measuring circuit used in these experi-

ments is shown in some detail in Fig. 5. It is a reasonably

sim.ple dc potentiometric circuit of the "substitution" type,

in which the voltage across the sample resistor is compared

with the voltage across a standard resistor. In this

measurement scheme, slow drift of the potentiometer standard

cell does not effect the accuracy of the measurement. Other

refinements adopted here are the temperature stabilizing of

the potentiometer working cell and of the resistor current

18

supply cell, the provision of continuous current drain

through a l-megohm resistor to electrically stabilize the

latter cell, and a high-quality four-deck switch (S4) which

simultaneously reverses the potentiometer current and the

resistor current.

In use, the circuit involved the selection of a partic-

ular germanium thermometer and current with switches S3 and

SI, respectively, and the sequential measurement of the

standard resistor and germanium thermometer voltages. The

use of reversing switch S4 during a voltage measurement

enabled the operator to ignore thermal emfs generated in

the voltage leads; since these would not change sign on

reversal of the current, they simply resulted in a zero

offset of the detector meter. Thermal emf ' s of one to

three microvolts were commonly seen in measuring the german-

ium thermometer voltages. The ratio of thermometer voltage

of standard resistor voltage provided the thermometer resis-

tance in terms of the standard resistance. The standard

resistor voltage drifted during a day's measurements by

one or two parts in ten thousand, and it was assumed that

this change represented drift in the resistor current.

Different current values were used for measuring the

different germanium thermometers. These currents ranged

from 0.1 yA to 10 yA, and in general we used the highest

current setting consistent with the avoidance of noticeable

heating of the geinnanium thermometer. This level was

easily determined by measuring the thermometer resistance

with several current values.

It is quite possible that the use of an ac resistance

measurement technique would have yielded results as accurate

as Cor, perish the thought, more accurate than!) the dc

technique described here; certainly the ac measurements are

faster and more sensitive. However, the relative simplicity

of the dc technique and its freedom from unknown sources of

19

error gave the present authors an extremely comforting

sense of well-being.

VII . REFERENCES

[1] International Committee on Weights and Measures, Metro-logia 5, 35 (1969)

.

[2] H. H. Plumb and G. Cataland, Metrologia 2^, 127 (1966).

[3] R. A. Kamper, paper Ml in Symposium on the Physics ofSuperconductive Devices Univ. of Virginia, 28-29 April,1967. See also R. A. Kamper and J. E. Zimmerman, J.Appl. Phys. 42^, 132 (1971).

[4] P. M. Berglund, H. K. Collan, G. J. Enholm, R. G. Gyllingand 0. V. Lounasmaa, J. Low Temp. Phys. 6_, 357 (1972).

15] See, for example, A. C. Anderson, RSI £1, 1446 (1970).

16] J. F. Schooley and R. J. Soulen, Jr., paper S-12, 5thSymposium on Temperature, 21-2 4 June 19 71, Washington,D. C.

17] R. J. Soulen, Jr. and J. H. Colwell, J. Low Tern. Phys.5, 325 (1971)

.

18] R. E. Fassnacht and J. R. Dillinger pointed out in Phys.Rev. 164 , 565 (196 7) that one can establish a minimumfield at the sample by varying the Helmholtz coilcurrent until a maximum T is reached.

c

19] R. H. Sherman, S. G. Sydoriak, and T. R. Roberts, J.Res. Nat. Bur. Stds. 6 8A, 579 (1964).

110] J. M. Daniels and F. N. H. Robinson, Phil. Mag. 44 ,

630, (1953-) .

Ill] It should be noted that a small shift in the fixed-point resistance did not render a resistor useless inthese experiments. This is true, once again, becausea group of fixed-point samples is present; in case acluster of AT^^ values larger than, say, 0 . 4 mK occurswith only one resistor, a new value of R^vE ^®adopted for that experiment such that the new AT^values cluster near the average AT^_. determined fromthe remaining thermometers. Then any single samplewhich shows an anomalous value of AT^ on the remain-ing resistors should show one of similar magnitudeand sign on the corrected resistor.

20

He RETURN

— He DEWARVACUUM JACKET

-He POT

'He PUMP LINE

e IMPEDANCE

',He EVAPORATOR

COUNTER CURRENTCAPILLARY

SUPPORT CYLINDER

DILUTION CHAMBER

Ce THERMOMETERS

PLATFORM

SUPERCONDUCTIVEFIXED POINTDEVICES

THERMAL LINK

PLATFORMHEATER

Figure 1. The experimental cryostat. The ^He return flowis equilibrated at 4.2 K, at 1 K, and at the sintered Cuflow impedance at 0.6 - 0.8 K. The impedance permits a flowrate of about 0.2 cc of '*He gas per minute with a 1-atmpressure differential. The heat exchanger is composed ofone meter of concentric .010" ID - .02" OD and .040" ID -

.050" OD stainless steel tubes. The ^He evaporator and thedilution chamber volumes are each about 5 cc, permitting awide range of operating conditions without displacing thephase boundary from the dilution chamber. The supportcylinders are two layers of .005" mylar. The platform is a6 cm dia. , 1 cm thick block of OFHC copper, and it is ther-mally connected to the dilution chamber by 100 #38 copperwires. The .004" electrical lead wires are thermally anchoredto the platform and to copper blocks on the dilution chamber,on the evaporator, on the He pot, and on the vacuum jacket.

21

Figure 2. Schematic of the copper disk and sample mountingassembly. The mutual inductance coils are wound on bake-lite formers. The primary coil contains 400 turns of #38AWG copper wire and is 2.5 cm long, while the 1 cm long,secondary coil contains 2000 turns of #40 AWG copper wire.The insert at the bottom shows the location of the individ-ual samples relative to the indicator notch in the copperstud.

22

Y-AXIS

LOCK- IN

DETECTOR

MUTUALINDUCTANCE

BRIDGE

SAMPLECOILS —

X-Y RECORDER

X-AXIS

DETECTOR

POTENTIOMETER

I COPPER DISC

DEWAR

GeTHERMOMETER

Figure 3. Block diagram of the transition measurementscheme, illustrating the definitions of the transition width,W. and of the transition temperature, T .

23

PHASE SENSITIVE DETECTOR

RECORDER OUT_

SIGNAL IN -OSCILLATOR

OUT

MUTUAL INDUCTANCE BRIDGEA B CDF

RECORDERY-AXIS

6 L

SAMPLE COILS

( TWO OF THE FIVE ARE SHOWN )

Figure 4. Schematic drawing of the mutual inductance circuit,A and F are isolation transformers, B is a 50 mH standardinductor, C is a ratio arm transformer, D reverses x'^ theinductive voltage component, and E reverses x" r the resis-tive voltage component. Switch G selects the appropriatesecondary coil.

24

NULL DETECTORSIGNAL IN RECORDER ^

OUT

RECORDERX-AXIS

POTENTIOMETERDETECTOR OUT ^^F

-WORKING CELL

S4

Figure 5. Schematic drawing of the resistance measurementcircuit. B is an oil-bath-stabilized mercury cell with acontinuous current draining resistor of 1 megohm, C is thepotentiometer constant current supply, D is a set of resis-tors which allow various thermometer currents to be used,R is a 1000 Q standard resistor, SI switches the thermometercurrent from one germanium resistor to another, S2 switchesthe potentiometer emf terminals from one resistor to anotherS3 is the current range switch, and S4 simultaneouslyreverses the potentiometer supply current and the resistorcurrent.

25

^ORM NBS-114A (1-71)

U.S. DEPT. OF COMM. 1. PUBLICATION OR REPORT NO. 2. Gov't Accessicwi

BIBLIOGRAPHIC DATA No.

SHEET NBS SP 260-44

3. Recipient's Accession No.

4. TITLE AND SUBTITLE Standard Reference Materials:Preparation and Use of Superconductive Fixed PointDevices, SRM 767

5. Publication Date

December 19726. Performing Or^nization Code

7. AUTHOR(S)

J. F. Schoolev, R. J. Soulen, Jr.. and G. A. Evans ^ Ji

8. Performing Organization

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDSDEPARTMENT OF rOMMFRTFWASHINGTON, D.C. 20234

l"0. Project/Task/Work Unit No.

11. Contract/Grant No.

12. Sponsoring Organization Name and Address

Same as block 9

.

13. Type of Report & PeriodCovered

Final14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

16. ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significantbibliography or literature survey, mention it here.)

The preparation, testing, and use of SRM 767 devices are described.

These devices incorporate samples of lead, indium, al\aminum, zinc, and

cadmium within a mutual inductance coil pair. These elements become

superconductive at temperatures near 7.2 K, 3.4 K, 1.2 K, 0.85 K and

Q.5 K, respectively, and the transition midpoints, when attained by

observing the sample magnetic susceptibilities in negligible small

magnetic fields, provide thermometric reference points which are

reproducible to + 1 mK.

17. KEY WORDS (Alphabetical order, separated by semicolons) _ - . _ .Aluminum; cadmium; cryogenics; indium;lead; magnetic susceptibility; superconductive transition temperature;superconductivity; ^h(=>r•TnnTn<a^-r^^ r' fivor^ points

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